The objective of this proposal is to extend the characterization of the early functions in the infective process of Rous sarcoma virus, an avian tumor virus. Conditional RSV mutants will be isolated which are deficient in viral products required early after infection. These mutants will be analyzed in order to ascertain whether there are any viral functions, other than the enzyme reverse transcriptase, necessary early in infection for both viral replication and cell transformation. The approach will be to mutagenize stocks of RSV and, using semi- selective conditions, isolate temperature-sensitive mutants with lesions expressed early in infection. Such mutants will then be characterized in temperature shift and biochemical experiments to determine the nature of the lesion. Three-factor genetic courses will be done to determine whether new mutants are identical to known mutants and to analyze the complexity of the RSV genetic map.